This invention relates to apparatus for indicating the level of liquid in a container particularly the level of molten metal in a continuous casting mold.
This invention is an improvement over that of U.S. Pat. No. 3,399,568 Wilson which in turn was an improvement in one embodiment of the invention of U.S. Pat. No. 3,204,460 Milnes, both of common ownership with this application. Wilson is directed to malfunction of the indicator when the metal level in the mold reaches the vicinity of the uppermost thermocouple. When this occurs the emf at the top thermocouple exceeded that of the bottom couple. This produced a zero balance point which caused the pen to be driven to a point falsely indicating the metal level was at the bottom of the measurement range instead of the top. A solution to this problem was provided by a reference probe connected to the mold wall at one end and the top point on the slide wire resistor so as to always provide a reference voltage at that point lower than the top thermocouple. However, there still have been occasions when the indicator malfunctions.
Investigation of this problem indicated that these situations occur when the emf profile is displaced from its normal values. As indicated in FIG. 1 when the profile is substantially displaced the difference between the maximum emfs in the region where the metal level actually is located and the balance point may be small. In this situation the normal `bias` built into the circuit (to force the pen to the low end of the scale when no signals are present) is larger than the difference between the maximum emf and balance point, thus causing the pen to travel full scale indicating a molten metal level at the bottom of the measuring range instead of the actual metal level.
I have found that the displacement of the emf profile is due to substantial temperature variations at the cold junctions of the thermocouples behind the casting mold. The temperatures at this location can be so high as to sometimes approach the temperature in the interior of the mold.
Several methods of correcting for cold junction temperature variation have been used in the past and are described in Metals Handbook 1948 edition pages 175 and 176. For example, one method is use of longer thermocouple or extension wires having equivalent thermoelectric characteristics instead of normal conductor lead wires. This removes the cold junctions to a more uniform temperature environment but is not always practical. Thermocouple wire is expensive and very long lengths may be required. Another method is to measure the temperature at the cold junctions and enter a calibration factor to an automatic compensator. This, however, is not practical in the present case, since it would overly complicate the level measurement circuitry. I have found a simple modification which overcomes the above disadvantages.
It is then a primary object of this invention to provide an improved liquid level indicating apparatus which will either eliminate or cancel the effect of substantial variation in temperature at thermocouple junctions adjacent an exterior face of the container.